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.TH GETENTROPY 3 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

getentropy \- fill a buffer with random bytes

.SH SYNOPSIS

.B #include <unistd.h>

.PP

.BI "int getentropy(void *" buffer ", size_t " length );

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR getentropy ():

.br

.RS 4

.ad l

_DEFAULT_SOURCE

.RE

.ad

.SH DESCRIPTION

The

.BR getentropy ()

function writes

.I length

bytes of high-quality random data to the buffer starting

at the location pointed to by

.IR buffer .

The maximum permitted value for the

.I length

argument is 256.

.PP

A successful call to

.BR getentropy ()

always provides the requested number of bytes of entropy.

.SH RETURN VALUE

On success, this function returns zero.

On error, \-1 is returned, and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EFAULT

Part or all of the buffer specified by

.I buffer

and

.I length

is not in valid addressable memory.

.TP

.B EIO

.I length

is greater than 256.

.TP

.B EIO

An unspecified error occurred while trying to overwrite

.I buffer

with random data.

.TP

.B ENOSYS

This kernel version does not implement the

.BR getrandom (2)

system call required to implement this function.

.SH VERSIONS

The

.BR getentropy ()

function first appeared in glibc 2.25.

.SH CONFORMING TO

This function is nonstandard.

It is also present on OpenBSD.

.SH NOTES

The

.BR getentropy ()

function is implemented using

.BR getrandom (2).

.PP

Whereas the glibc wrapper makes

.BR getrandom (2)

a cancellation point,

.BR getentropy ()

is not a cancellation point.

.PP

.BR getentropy ()

is also declared in

.BR <sys/random.h> .

(No feature test macro need be defined to obtain the declaration from

that header file.)

.PP

A call to

.BR getentropy ()

may block if the system has just booted and the kernel has

not yet collected enough randomness to initialize the entropy pool.

In this case,

.BR getentropy ()

will keep blocking even if a signal is handled,

and will return only once the entropy pool has been initialized.

.SH SEE ALSO

.BR getrandom (2),

.BR urandom (4),

.BR random (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.